Aesop's Fables, translated by Laura Gibbs (2002)

A fly alighted on a wagon pole and began to harass the mule. 'You are
moving so slowly!' she said, 'Can't you walk any faster than this? Watch
out, or I will pierce your neck with my sting!' The mule replied, 'I don't
care what you say: the only thing that scares me is that man who sits
up there in front regulating my pace with his pliant whip and keeping
my head in check with the foaming bit. I've had enough of you and your
silly boasting: I know full well when I am supposed go slowly and when
I am supposed to run!' This fable can be effectively used to ridicule a person who makes empty
threats without having the power to back them up.

Source:
Aesop's Fables. A new translation by Laura
Gibbs.
Oxford University Press (World's Classics): Oxford, 2002.
NOTE: New
cover, with new ISBN, published in 2008; contents of book unchanged.